Field of the Invention
The invention relates to an apparatus and method for illuminating tissue.
Related Art
Optogenetics is an emerging new field in neuroscience, in which methods for controlling and readout of neural activity are being developed. As the field matures the need for advanced technological tools for projecting patterned light into deep brain areas is becoming clear. Ideally, the projected light should have temporal resolution on the order of 1 millisecond and spatial resolution on the order of the size of a neural cell body.
Current most advanced technologies for high-resolution light projection into neural tissues are based of free-space optics. These technologies can only address superficial layers of the brain. Implantable technologies should be considered beyond that depth.
There are currently three fundamental limitations that prevent current implantable technologies from projecting high spatially patterned light in deep tissue areas. First, the light delivery method into deep tissue area might be bulky, and thus cannot be packed in a dense matter. Second, the light generation itself can have some limitations (for example heat generation in μLEDs), which prevents dense packaging and operation of many emitters together. Third, the illumination pattern generated by a single illumination point might be spatially broad, thus forcing the physical separation of the emitters in order to maintain good contrast between them. A mixture of these limitations exists in current methods for light delivery into deep tissue areas, and therefore they provide illumination patterns having only very low spatial resolution.
Currently there are no available technologies for projecting high-resolution illumination patterns into a biological tissue beyond a depth thicker than about 1 mm. Free-space projection methods cannot tightly focus the light at deeper depths due to the scattering properties of the biological tissues. This is true for the vast majority of the biological species, excluding very few, which are transparent to light. For this vast majority, implantable projection methods are therefore required in order to project high-resolution light at arbitrary depth beyond 1 mm. To clarify, the resolution of an illumination pattern in a biological tissue is measured relative to the size of the targeted cell bodies in that tissue. A high-resolution illumination pattern would be one in which the density of the illuminating pixels and the shape of the illuminated beam by those would have this characteristic size throughout the effective volume of illumination.
Most of the current excising implantable probes for projecting light into deep tissue areas lack the ability to deliver high-resolution patterns into those areas for reasons, which are even more fundamental than the shape of the illuminated beam created a single pixel comprising these projection devices. One example is of probes utilizing optical fibers for light delivery into deep tissue areas. The size of an individual fiber is already much larger than the size of a cell body, thus excluding the possibility that this method would be used for high-resolution illumination. Collimating the light coming out of a fiber would require even a bulkier construction on top of that.
Another example method utilizes mode division multiplexing in order to generate several illumination points addressable through each optical fiber. The individual addressing of each illumination point by this method, however, requires separating adjacent points by a rather large distance. Therefore, also in this example, the shape of the beams does not limit the resolution of the illuminated pattern.
A third example comprises of implantable devices that utilize μ-LEDs as emitting pixels for generating patterned illumination. Currently, for some application in which thermal restriction apply, the resolution of these devices is limited by the access heat production of the μ-LEDs, which prevents the packaging of more than very few μ-LEDs on a single implanted device. For applications in which thermal restriction is less of a problem, the resolution would be limited by the fact that the light emitted by μ-LEDs is not collimated. The resulting illumination beam experiences rapidly decaying power intensity as a function of the distance from the illumination point. In addition, the expansion of the beam creates crosstalk between illumination beams. Namely, adjacent illumination beams can overlap and partially illuminate the same tissue volume. In that case the distance between the illumination points needs to be increased, resulting in reduction of the illumination resolution.